1. Field of the Invention
The invention relates generally to heat sinks and manufacturing methods thereof; and more particularly to a kind of heat sink which conducts heat by employing carbon nanotubes, and a manufacturing method thereof.
2. Description of Related Art
Electronic components such as semiconductor chips are becoming progressively smaller, while at the same time heat dissipation requirements thereof are increasing. Commonly, a thermal interface material is utilized between the electronic component and a heat sink in order to dissipate heat generated by the electronic component.
A conventional thermal interface material is obtained by diffusing particles with a high heat conduction coefficient in a base material. The particles can be graphite, boron nitride, silicon oxide, alumina, silver, or other metals. However, the heat conduction coefficient of the thermal interface material is now considered to be too low for many contemporary applications, because it cannot adequately meet the heat dissipation requirements of modem electronic components.
A new kind of thermal interface material has recently been developed. The thermal interface material is obtained by fixing carbon fibers with polymer. The carbon fibers are distributed directionally, and each carbon fiber can provide a heat conduction path. A heat conduction coefficient of this kind of thermal interface material is relatively high. However, the heat conduction coefficient of the thermal interface material is inversely proportional to a thickness thereof, and the thickness is required to be greater than 40 micrometers. In other words, the heat conduction coefficient is limited to a certain value corresponding to a thickness of 40 micrometers. The value of the heat conduction coefficient cannot be increased, because the thickness cannot be reduced.
In 1991, carbon nanotubes was discovered. A length of a carbon nanotube can be several thousand times its diameter. A tensile strength of carbon nanotubes can be one hundred times that of steel. A weight of carbon nanotubes is only one sixth of that of steel. Furthermore, carbon nanotubes have good flexibility and excellent heat conduction coefficients. Thus, carbon nanotubes have excellent characteristics for thermal conduction applications. An article entitled “Unusually High Thermal Conductivity of Carbon Nanotubes” and authored by Savas Berber (page 4613, Vol. 84, Physical Review Letters 2000) discloses that a heat conduction coefficient of a carbon nanotube can be 6600W/mK (watts/milliKelvin) at room temperature.
U.S. Pat. No. 6,407,922 discloses a thermal interface material employing carbon nanotubes. The thermal interface material is formed by injection molding, and has a plurality of carbon nanotubes incorporated in a matrix material. A first surface of the thermal interface material engages with an electronic device, and an opposite second surface of the thermal interface material engages with a heat sink. The second surface has a larger area than the first surface, so that heat can be uniformly spread over the larger second surface.
However, the thermal interface material is formed by injection molding. The thermal interface material is relatively thick, and has reduced flexibility. In addition, the carbon nanotubes are disposed in the matrix material randomly and multidirectionally. This means that heat does not necessarily spread uniformly through the thermal interface material. Furthermore, heat does not necessarily spread directly from the first surface engaged with the electronic device to the second surface engaged with the heat sink.
A new heat sink with carbon nanotubes which overcomes the above-mentioned problems and a method for manufacturing such heat sink are desired.